Physical Sciences Division Research Highlights

Simulation, Calculations Show Hydroxide Ions Orientation in Water

Hydroxide ions use two-step process to form two different structures

Studying hydroxide ions in water, the team found that the ion (center) can form two different structures: one with five hydrogen bonds (left) and the other with four. Enlarge Image

Results: Whole water molecules form complex shapes around hydroxide ions, simple negatively charged particles, according to a recent study by scientists at Pacific Northwest National Laboratory. The shapes are the result of hydrogen bonds between the ions and the molecules. This research answers the question, debated in scientific circles for more than 70 years, of how hydroxide ions get oriented in water. This work graced the cover of the October 19, 2009, issue of Chemical Physics Letters.

Why it matters: Knowing how hydroxide ions are arranged in water could aid scientists in fine-tuning current industrial processes, such as manufacturing biodiesel, making processes more efficient or less wasteful. Further, it could assist in developing future industrial processes, such as turning poplar trees and other vegetation into automotive fuel.

“However, it isn’t all about applications,” said Dr. Liem Dang, the principal investigator on the study. “It’s about understanding, fundamentally, how our universe works.”

Methods: The theoretical chemistry team from Pacific Northwest National Laboratory built and ran simulations of how hydroxide ions and water molecules moved based on the basic principles of physics. These simulations took into account the mutable shape of the electron clouds around each atom. These models are called classic molecular dynamics simulations with a polarizable force field.

Studying the ions in water, the team found the hydroxide ion can form two different structures. In the most common motif, created about 57% of the time, hydroxide’s oxygen atoms bonded with the hydrogen atoms on 4 water molecules while the hydroxide’s hydrogen bonded to the oxygen atom on another water molecule. The structure looks like a pyramid with a square base. In the other motif, formed about 38% of the time, hydroxide’s hydrogen atom refused to bond to any water molecules.

The researchers determined that the structures are created via a two-step process. First, the water molecules around the hydroxide ion bounced around, positioning and creating hydrogen bonds with the hydroxide. Once the structure was formed, the hydroxide ion relaxed to the minimum energy configuration.

What’s Next: The scientists continue to examine the behavior of hydroxide ions at the water/air interfaces with molecular dynamics simulations and other approaches. The knowledge they gain will help control complex chemical processes.

Acknowledgments: This research was done by Xiuquan Sun, Soohaeng Yoo, Sotiris Xantheas, and Liem Dang of Pacific Northwest National Laboratory. This research used NWChem at DOE's EMSL, a national scientific user facility located at PNNL.